As flexible organic electronic devices become more of a commodity, the need for multifunctional composite encapsulating films has risen sharply. The form factor of these next generation devices are offering new challenges in packaging and barrier film technology. A typical flexible organic light emitting diode (OLED) display is on the order of several hundred microns in thickness. Considering that the organic active component of the OLED readily deteriorates with trace amounts of water and/or oxygen, highly robust, optically transparent, flexible films are required to hermetically seal the device from the ambient atmosphere containing water and oxygen.
Permeation of gases through a barrier is governed by both sorption into and diffusion through the barrier matrix. It has been firmly established that permeation of heavy gases is a diffusion-controlled process while diffusion of light gases, such as O2 and H2O, is a sorption-controlled process1 due to the high vapor pressure of light gases2. It is therefore of outmost interest to reduce both sorption into and diffusion through the polymer matrix. Water-repelling barriers, e.g. perfluorinated polymers3 may be used to reduce the sorption into the polymer matrix.
Ceramic glass has been the mainstay for hermetic sealing of electronically sensitive devices which require an “absolute” barrier from the ambient environment. The limitation is that the typical inorganic glasses do not have elastomeric character leading to flexible, ultra thin mechanically stable structures. The ideal material would have the absolute gas barrier characteristics of ceramic glass with the proccessability and formability of a room temperature liquid solution which cures well below the working temperature of ceramic glass, typically between 800 and 1100° C. for standard borosilicates.
Recent work has been devoted to formulating such materials. These materials are commonly known as ORMOCERS4a,b, CERAMERS5, and Hybrid Glass6, sometimes called spin-on glass. These materials impart the physical properties of ceramic glass, i.e. high temperature durability, robust mechanical properties and chemical resistance which can be manipulated and handled as room temperature liquids or viscous slurry prior to cross-linking or curing. After cross-linking, the material has enhanced heat resistance rising from the oxide network and shows flexibility resulting from the covalent organic moieties or intercalated organic network.
If such a material were available for mass production today, a single layer deposition would not be enough to ensure hermetic sealing. The reason being is process variances and substrate inhomogeneity induced defects. Combining these two critical parameters in high throughput processing has revealed defects ranging from the nanometer to the micron scale7a-d . The defects, depending on their proximity per unit area to one another, will negatively influence the mass transfer, or transmission, of ambient gas inward toward the active layer of the device. Therefore, multiple layers of an active barrier material must be deposited to effectively increase the diffusion path length of the gas. It has been shown that buffering the active barrier material between polymer layers is effective to minimize the effect of deposition process induced defects by sufficiently increasing the diffusion path length. A sufficiently long diffusion path length will reach equilibrium, or steady state, conditions before the active layer of the device is impeded.
A typical architecture of commercial barrier films are alternating layers of amorphous inorganic networks like silicon dioxide or silicon oxynitride between acrylic polymer films on a polyester substrate. These materials are successively sputtered by high vacuum processes. The polymeric layer is deposited between the amorphous inorganic films producing a series of alternating layers called dyads. A film can consist of one or many dyads depending on the desired permeation rate of water and/or oxygen.
Prior art approaches to solving the sorption-diffusion problem have included using, Mitsubishi Plastics'8a-i Xbarrier™, and Thin Film Inorganics on PET such as Barix™ by Vitex Systems, Inc7b-d.